Let $f: R \rightarrow R$ be a differentiable function having $f(2)=6,…

Let $f: R \rightarrow R$ be a differentiable function having $f(2)=6, f^{\prime}(2)=\left(\frac{1}{48}\right)$. Then $\lim _{x \rightarrow 2} \int_6^{f(x)} \frac{4 t^3}{x-2} d t$ equals
  1. 24
  2. 36
  3. 12
  4. 18

Solution

$ \lim _{x \rightarrow 2} \int_0^{f(x)} \frac{4 t^3}{x-2} d t $ Applying $L$ Hospital rule $ \begin{aligned} & \lim _{x \rightarrow 2}\left[4 f(x)^2 f^{\prime}(x)\right]=4 f(2)^3 f^{\prime}(2) \\ & =4 \times 6^3 \times \frac{1}{48}=18 . \end{aligned} $

Asked in: JEE Main 2005

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